Eight months after announcing its acquisition of The Block on Congress, an historic five-building grouping of office and retail space in Boston’s financial district, Related Beal has launched the transformation of the 343,000-square-foot space into the mixed-use Congress Square.

Related Beal’s plans for Congress Square, which spans an entire city block, entail a mix of the old and the new, a live-work-play destination featuring premier office and retail space encased in the architecturally significant structures along Water, Devonshire and Congress streets. And there’s also developable land to play with at 54 Devonshire St.

Related Beal has tapped integrated design firm Arrowstreet to spearhead the makeover of the buildings, the oldest of which was developed in 1899. Ultimately three of the structures will be marketed to specialized developers for conversion. And one of the most important changes to the block will be the development and activation of Quaker Lane, essentially an alley that runs through the middle of the property, which will link the Post Office Square and the high-traffic Faneuil Hall.

“Our plan is to rejuvenate the entire block through a mixed-use redevelopment strategy,” Stephen Faber, executive vice president with Related Beal, noted in a prepared statement. “The market is looking for efficient, modern space, rich with amenities and energy. These buildings and this location present us with the opportunity to deliver a variety of uses, bring activity and retail to Post Office Square, and create a real destination in Quaker Lane.”

Commercial real estate services firm JLL is onboard to spearhead leasing of the office space, while CBRE/Grossman Retail Advisors will work with retail users. Congress Square’s retail segment will debut first, in the fall of 2015, followed by the opening of the office portion in early 2016.